With a small spoon add the cheesy mashed potato mixture into the cavity of each mushroom, squeezing in as much as you can.

When the onions are finished caramelizing, carefully scoop a little on to the top of each mushroom cap. Place stuffed mushrooms in the oven and bake for 20 minutes. Remove from the oven and top each mushroom cap with a little extra parmesan and fresh herbs and continue to cook for an additional 15-20 minutes.